An industrial process control transmitter is a transducer that responds to a process variable and provides a standardized transmission signal as a function of the measured variable. The term "process variable" refers to a physical or chemical state of matter or conversion of energy. Examples of process variables include pressure, temperature, flow, conductivity, pH, and other properties. One such transmitter is an industrial process pressure transmitter used to measure flow (difference of two pressures), level (head or back pressure), and temperature (fluid pressure in a thermal system). Such transmitters are used in industrial processes, such as with slurries, liquids, vapors and gasses in chemical, pulp, petrochemical, gas, pharmaceutical, food, and other fluid processing plants.
Process control transmitters are often operated in hazardous field environments and are connected by two-wire communication lines to a central or control station. Typically, the central or control station additionally supplies operating power to the remote process control transmitters on the two-wire communication line. A typical two-wire system permits both analog and digital communications between the central station and the remote transmitter. Typically, the analog communications is performed between 4 and 20 microamperes (mA). One well-known digital communications system for two-wire process control systems is the HART.RTM. system, commercially available from Rosemount Inc. of Eden Prairie, Minn. The HART system provides communication between the central station and the remote field transmitter.
The HART system uses a half-duplex master-slave protocol. Typically the master sends a command and then expects a reply; the slave waits for a command and sends a reply. Each command or reply is a message, varying in length from a few bytes to as many as 30 bytes. The message consists of asynchronous serial data transmitted at 1,200 bits per second (BPS). Transmission is accomplished through frequency-shift-keying (FSK) such that a logical 1 is represented by a 1,200 Hertz (Hz) signal and a logical 0 is represented by a 2,200 Hz signal. These HART protocol signals are modulated onto the two-wire communications line carrying the D.C. power.
Using the HART communication protocol, a message transaction is initiated by the central or control station sending a command to a specified field transmitter, usually by addressing the field transmitter with a unique address. The command may, for example, direct the transmitter to reply with information as to its status, such as current pressure sensed, etc. Upon receipt of the command from the control station, the transmitter sends a reply to the control station which is received and processed. (In some cases, the transmitter may be programmed to send a reply repeatedly without need for the control station to send a command each time.) The HART system permits the control station to conduct approximately two or three transactions per second. If a large number of field transmitters are connected to the control station by the same two-wire communication channel, the limited number of transactions limits the communication volume, which in turn limits the control functions that might be achieved. Consequently, Rosemount Inc. developed a communications system, called "HSH", having a greater volume that permits a greater number of transactions.
HART signals are non-coherent FSK signals that are demodulated by first mixing with a free-running local oscillator frequency of 1700 Hz, and then identifying the phase of the resultant signal. Mixing the HART signals with the 1700 Hz frequency shifts the 1,200 Hz (binary 1) and 2,200 Hz (binary 0) signals to +/-500 Hz signals. The phase of the resultant signal will either continuously increase, thus indicating that the resultant signal was derived from the 1,200 Hz signal, or the phase will continuously decrease, thus indicating that the resultant signal was derived from the 2,200 Hz signal. Bit recognition of the HART protocol is achieved by sensing whether the phase is increasing (binary 1) or decreasing (binary 0).
The HSH system employs a bandwidth-efficient technique known as 8-ary phase shift keying (8PSK) that allows transmission of symbols at the rate of 3,200 symbols per second. The carrier is shifted among eight possible phases, or symbols, each of which represents three binary bits. Consequently, the 3,200 symbols per second rate yields a bit rate of 9,600 BPS. HSH decodes the in-phase and quadrature amplitudes of the coherent 8PSK signal to identify the symbol, and hence the three-bit representation. HSH signals are demodulated by mixing the signal with a coherent local oscillator at a frequency of 3200 Hz. Thus, the HART and HSH protocols require different demodulation techniques for receiving and demodulating signals.
The HART protocol includes a preamble followed by the user message. The preamble is used to set the gain of gain control circuits to assure proper signal strength to the receiver, and is used to synchronize the local oscillator to the bit rate of the message. The HSH protocol also includes a preamble of certain 8PSK symbols that set the gain of the gain control circuits, lock a phase lock loop onto the 8PSK carrier frequency and establish operating parameters of an equalizer filter. The messages in both HART and HSH protocols are reasonably short. The HART protocol messages usually are no greater than 30 bytes, with a maximum duration of 200 milliseconds (msec). The HSH protocol messages employ preambles that are typically 64 symbols, followed by the user message of up to 128 symbols, representing 48 bytes of user data requiring a maximum duration of 60 msec. Hence, the HSH protocol permits a greater number of transactions per second, and larger user messages, than the HART protocol.
A large number of transmitters currently in the field employ the HART protocol. Accordingly, it is economically unfeasible to convert those transmitters to HSH protocol. Moreover, the HART protocol is adequate in many field environments, such as in control systems not requiring a large number of field transmitters or a large volume of messages. Nevertheless, as the smaller control systems become more complex, it is desirable to use the HSH protocol in certain circumstances. Accordingly, there is a need for a single receiver system that automatically detects the incompatible communication protocols and demodulates the signals in accordance with the detected protocol. In particular, there is a need for a single circuit that is capable of demodulating signals in both HART and HSH protocol, requiring minimal number of functional blocks, thereby minimizing current consumption and cost.